In an image input apparatus, such as a digital camera, for capturing an image using a solid-state image sensing device, exposure is performed by opening and closing a mechanical shutter.
In a case where use is made of an image sensor such as an interlaced-readout interline (IT) CCD, frame-readout (FT) CCD or CMOS sensor, the general practice is to set the sensor to a charge accumulation state and open and close the mechanical shutter in this state, whereby exposure is performed.
The structure of a prior-art image input apparatus is illustrated in FIG. 12 (for example, see the specification of Japanese Patent Application Laid-Open No. 2003-101873). FIG. 12 illustrates the structure of a so-called focal-plane shutter-type arrangement in which the shutter is disposed in the vicinity of the focal plane of the lens.
Although light from a subject forms an image on a solid-state image sensing device 6 via a lens 1, it is possible to shield the solid-state image sensing device 6 from the light from the subject by the shutter 2. The shutter 2 is controlled by a shutter drive unit 5. The shutter 2 comprises front and rear curtains 3 and 4, respectively. The solid-state image sensing device 6 is driven by a sensor driving circuit 7 and is controlled in a charge accumulation mode for accumulating electric charge generated in accordance with the amount of light that is incident on the photoreceptor surface of the solid-state image sensing device 6, and in a readout mode for reading out the electric charge accumulated. Control for driving the solid-state image sensing device 6 in these modes is exercised by a CPU 8.
FIG. 13 is a diagram illustrating the structure of the solid-state image sensing device 6.
The solid-state image sensing device 6 includes a plurality of pixels 14. A line is formed by disposing a prescribed number of the pixels 14 horizontally, and a prescribed number of those lines are disposed vertically on different lines, thereby a two-dimensional sensor is constructed. Peripheral circuitry (not shown) is disposed at the periphery of the sensor, thereby constructing the solid-state image sensing device 6.
FIG. 14 illustrates the timing for driving the shutter 2 and solid-state image sensing device 6.
The solid-state image sensing device 6 is set to the charge accumulation mode by a sensor driving circuit 7 and travel of the front curtain 3 is started by the shutter drive unit 5. Travel of the rear curtain 4 is started by the shutter drive unit 5 upon elapse of S seconds. At completion of travel of the rear curtain 4, the solid-state image sensing device 6 is placed in the readout mode by the sensor driving circuit 7 and the accumulated electric charge (image data) is read out. At this time, each pixel of the solid-state image sensing device 6 is irradiated with light for a period of time (S seconds) equivalent to the difference between the travel times of the front and rear curtains.
With the prior art described above, however, short exposure time for high-speed exposure is decided by mechanical limitations. In general, if use is made of a focal-plane shutter that transports a slit, the precision achieved on the high-speed exposure side owing to mechanical limitations is 1/4000 to 1/8000 sec.
On the other hand, in a case where use is made of a progressive-scan (total-pixel readout) ITCCD, the mechanical shutter can be opened in advance and control of exposure time by an electronic shutter can be performed. In the case of this arrangement, there are no mechanical restraints on the side of high-speed exposure. With use of an electronic shutter, it is possible to achieve a high-speed exposure of 1/16000 sec., etc., which is at least twice as fast as that when there is a mechanical limitation.
However, with an interlaced-readout ITCCD, FTCCD or CMOS sensor, it is impossible because of sensor structure to control exposure time at the above-described level on the side of high-speed exposure by an electronic shutter.